Production of aqueous zinc chloride electrolyte saturated with chlorine

ABSTRACT

An aqueous solution of zinc chloride is saturated with chlorine gas by flowing a zinc chloride solution through a saturator vessel in continuous flow from top to bottom thereof, passing chlorine gas in very finely divided form into the bottom of the vessel into contact with the moving zinc chloride solution so that chlorine not dissolving in the zinc chloride solution rises to the top of the vessel, and recirculating chlorine gas from the top of the vessel, mixing it with additional chlorine gas to be fed to the bottom of the vessel and passing recycle and feed chlorine gases into the zinc chloride solution in finely divided form at the bottom of the vessel.

United States Patent [1 1 Bjorkman {111 3,775,187 [451 Nov. 27, 1973Inventor:

Filed:

Appl. No.: 200,067

Harry K. Bjorkman, Birmingham,

Mich.

Assignee: Occidental Energy Development Company, Madison Heights, Mich.

Nov.

References Cited UNITED STATES PATENTS 7/1910 l/l960 l/l972 BenkoCrowley et al Setser et al. 136/86 A 3,227,585 [/1966 Langford et a]136/86 E Primary Examiner-A. B. Curtis Assistant Examiner-H. A. FeeleyAttorney-William J. Schramm [5 7] ABSTRACT An aqueous solution of zincchloride is saturated with chlorine gas by flowing a zinc chloridesolution through a saturator vessel in continuous flow from top tobottom thereof, passing chlorine gas in very finely divided form intothe bottom of the vessel into contact with the moving zinc chloridesolution so that chlorine not dissolving in the zinc chloride solutionrises to the top of the vessel, and recirculating chlorine gas from thetop of the vessel, mixing it with additional chlorine gas to be fed tothe bottom of the vessel and passing recycle and feed chlorine gasesinto the zinc chloride solution in finely divided form at the bottom ofthe vessel.

3 Claims, 3 Drawing Figures PRODUCTION OF AQUEOUS ZINC CHLORIDEELECTROLYTE SATURATED WITH CHLORINE BACKGROUND OF THE INVENTION In theoperation of high energy density secondary batteries based on zinc andchlorine-on-carbon electrodes, 'an aqueous zinc chloride solutionsaturated with chlorine is used as a feed to the anodes during thedischarging of the battery. Chlorine is converted to chloride ion at theanode and metallic zinc or other suitable metal on the surface of theother cathode is converted to the corresponding metal ion. The effluentfrom the battery is unsaturated with respect to chlorine and has to beresaturated before being recirculated to the same or similar batteryduring a continuing discharging operation. Thus, to keep such batteriesin operation, it is necessary to continuously resaturate an aqueous zincchloride solution with chlorine.

Difficulty has been noted in attempting to saturate zinc chloridesolutions, especially those of the more concentrated types generallyemployed in high energy density batteries. Often, using conventionalbubbling and mixing techniques, the solution rate is slow and onlypartially resaturated zinc chloride solution is returned to thebatteries, leading to lower efficiencies of power discharge. To overcomesuch difficulties larger volumes of saturator vessels have been used sothat the holdup time in the vessel may be lengthened. Now, however, ithas been discovered that such saturating operation, difficult as itoften is, can be speeded significantly by following the process of thisinvention.

SUMMARY OF THE INVENTION In accordance with the present invention, anapparatus for dissolving chlorine in an aqueous solution of zincchloride and saturating the solution with chlorine comprises a vessel orpassage through which the zinc chloride solution flows, a duct forcommunication of a source of chlorine with the flowing electrolyte and,between the duct and the vessel or passage, a means for breaking down astream of chlorine gas from the source of chlorine into a plurality ofvery thin streams of chlorine gas and chlorine gas bubbles, which breakoff therefrom as they enter the volume of the zinc chloride solution.With respect to the method aspect of the invention, it comprises amethod for dissolving chlorine in an aqueous solution of zinc chlorideand saturating the solution with chlorine by passing into a volume ofmoving zinc chloride solution very thin streams of chlorine gas andchlorine gas bubbles from bottom to top of such solution and into a zoneabove such solution, withdrawing undissolved chlorine gas from the zone,mixing it with other chlorine gas and admitting it to the bottom of thesolution in a plurality of very thin streams and bubbles, addingunsaturated zinc chloride solution to the top of the volume andwithdrawing saturated zinc chloride solution from the bottom thereof atsubstantially the same rate as solution is added to the volume.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention and itsmode of operation will be apparent from the following description, takenin conjunction with the accompanying drawing, in which:

FIG. 1 is a substantially schematic, partly sectional elevational viewof the apparatus of this invention,

showing flow of electrolyte through a saturator and its return to a highenergy density battery being discharged;

FIG. 2 is a cutaway plan of a glass frit sparger utilized to producechlorine bubbles passed through an electrolyate to be saturated withchlorine; and

FIG. 3 is a central vertical sectional view of such sparger along plane33 of FIG. 2.

A high energy density secondary battery or cell bank 11 includes aplurality of bipolar (or monopolar) electrodes I3 and electrolyte zones15 between the electrodes. In discharging electricity from the battery,electrolyte containing elemental halogen is passed into the batterythrough line 17 and inlet 19 and exits from the battery through outlet21 and line 23. In a battery having bipolar cells in which theelectrodes are of a highly electropositive metal, such as Group'IIB,VIII, especially zinc, and chlorine-on-carbon, the electrolyte isaqueous metal halide solution such as zinc chloride, usually at aconcentration of 0.1 to saturation, preferably 5 to 50 percent and evenmore preferably 10 to 50 percent zinc chloride, and contains chlorinedissolved therein, usually from 0.5 to 3 volumes of chlorine per volumeof electrolyte. As the electrolyte passes through the battery thechlorine is converted to chloride ion and zinc from the zinc electrodedissolves in the electrolyte, forming zinc ion. Thus, when theelectrolyte enters the cell through inlet 19 it contains dissolvedelemental chlorine (chlorine gas) and when it leaves through exit 21 itis depleted in chlorine content. Therefore, it must be resaturated withchlorine before being recirculated through the battery during thedischarge mode.

It will be noted that in FIG. 1 the representation of the battery isschematic and specific paths for electrolyte flow, other than agenerally vertical flow, are not shown. The electrolyte may pass througha porous carbon anode, as is described in an application identified asCase U10-033 Ser. No. 200,041, filed the same day as the presentapplication, entitled BIPOLAR ELEC- TRODE FOR CELL OF HIGH ENERGYDENSITY SECONDARY BATTERY, in which the present inventor is aco-inventor. Alternatively, flow may be in a cell containing a separatoror membrane between the electrodes and plural flows of electrolyte maybe utilized. Other cell types and various materials of construction aredisclosed in US. Pat. application Ser. No. 50,054 for HALOGEN HYDRATES',now US. Pat. No. 3,713,888, in which the present inventor is also acoinventor. In all such cases, it will still be necessary to resaturateeffluent from the cell or another source with chlorine so as to make thecell operative to supply electricity when the electrolyte is circulatedthrough it. When refuellable cells or replaceable electrodes are usedresaturation with chlorine will also have to be undertaken.

Depleted electrolyte 25 in line 23 is pumped by pumping means 27 intosaturator vessel 29 having upper and lower portions 31 and 33. Theelectrolyte, a 25 percent zinc chloride solution in water, having lessthan one volume of chlorine therein, enters vessel 29 at inlet 35 nearits top and flows downwardly through the vessel, dissolving finelydivided chlorine bubbles 37 until it is saturated, and exits from thevessel through outlet 39 past pump 27 and back to battery 11. As isillustrated, the entry is at one side of the vessel 29 but various entryports may be provided around the perimeter of the vessel or across thetop thereof. In a similar manner, plural outlets may be provided butthese are preferably located near the chlorine inlets so that acountercurrent flow effect is obtained and the greatest driving force ofchlorine concentration is available at the outlet to make sure that theelectrolyte is completely saturated with chlorine.

The chlorine circulation system comprises a source of chlorine, whichmay be pressurized tank 41, having an outlet valve 43 controllingchlorine flow through line 45 to spargers, diffusers or septa 47. Thechlorine flow through the spargers, which are preferably of glass,porcelain, titanium, or plastic frit or of perforated materials of thesetypes, is at a sufficient velocity to maintain the body of electrolyte25 in vessel 29 in a state of turbulent motion which promotes solutionof the chlorine gas in the electrolyte during the period in which theyare in contact. The spargers may be so located and directed as toproduce the best turbulent flows of electrolyte and chlorine streams orbubbles emitted from them. After most of the chlorine has dissolved inthe electrolyte, if any of it is still undissolved, it rises to section31 and forms a gaseous phase atop the electrolyte, which aids indissolving chlorine in the upper portion of the electrolyte, normally ofthe lowest concentration because it is closest to the inlet for depletedelectrolyte. From the top of the vessel the chlorine gas is recirculatedthrough line 49 back to line 45, where it mixes with additional chlorinebeing charged, and subsequently it is sent back through the spargers tobe dissolved in the electrolyte. Check valve means may be provided inline 49 and pumping or blowing means, not illustrated, may be utilizedto assure that the chlorine in the upper portion of the saturator vesselis mixed in with fresh chlorine and is recirculated through theelectrolyte.

A saturator vessel has been illustrated but it will be evident that thesame principles may be applied to pipes, ducts, tubes or otherpassageways, reservoirs, tanks and other containers in which contactbetween finely divided chlorine gas streams and bubbles with electrolytemay be made. In such cases it will be important to entirely dissolve thegas in the electrolyte or provide some means for recovering thechlorine, possibly at a high point in a line or duct downstream from themain resaturation operation, so that the chlorine may be recirculatedand not wasted.

The sparger of FIG. 2 is illustrated as circular and disc-like, havingan inlet entrance section 51 to convey the chlorine to the variousopenings 53 in the surface 55 of the sparger and thereby convert alarger stream of chlorine gas into a multiplicity of very fine streams,which break into bubbles 37 in the volume of electrolyte. A top 59 isshown, containing holes 57 therein, through which the bubbles pass. Thetop helps to maintain the integrity of frit or very fine passages, butwhen the porous materials are sufficiently stable and well bonded, thetop may be omitted. Of course, other shapes of spargers may be employedand they can be of various sizes, usually being from 1 cm. to cm. indiameter. Constructions of such diffusers are known, and involve thepartial fusion of many small pieces or frit of small materials, such asglass, porcelains and synthetic organic plastics, e. g.,polytetrafluorethylene, polyvinyl chloride, after-chlorinated polyvinylchloride, polymethyl methacrylate, polyethylene, polypropylene,chlorinated polyethylene and chlorinated polypropylone. Glass and Teflonare preferred sparger materials, the latter being especially usefulbecause of its hydrophobic nature. The frit sizes, before fusion, willusually have one dimension in the l to 100 microns range and the fritwill be fused together in such a way that the passages through it willbe in the l to 100 micron range, preferably from 5 to 25 microns. Thus,when chlorine is passed through entrance 51 and into the channels of thesparger, it will be divided into very thin streams and will be emittedfrom the sparger in such streams into the electrolyte volume. Due to thecirculation of electrolyte caused by the sparging forces and theelectrolytes flow, the stream will be broken up into bubbles, whichprovide greater surface areas to the electrolyte for absorption.

Instead of the spargers shown, septa and perforated discs of otherdesigns can also be used, providing only that the outlets therefrom aresufficient in number so that the pressure drop is not exceptionallyhigh, preferably not over 100 mm. of mercury at the rate of flowrequired for saturation of the electrolyte. It is also desirable thatthe passages or openings be of such construction that they do not becomereadily plugged or, if blocked, may be cleared by backwashing. Nobackwashing mechanism is shown but the methods of clearing blockedpassageways in diffusers are well known and need not be illustratedhere.

In operation, using the apparatus of FIG. 1 with the spargers of FlGS. 2and 3, electrolyte from battery 1 1, depleted in chlorine content, isfed to vessel 29 and is withdrawn from that vessel at about the samerate at which it is added, maintaining a constant level of electrolyteat about 65 percent of the volume of thevessel. The battery which is asource of electrolyte to be saturated with chlorine may be of cells ofthe types described in Case 2802 Ser. No. 200,047 for MANU- FACTURE OFCHLORINE HYDRATE, filed the same day as the present application, or US.Pat. application Ser. No. 50,054 for HALOGEN HYDRATES, in both of whichapplications the present inventor is a coinventor. Normally, such levelwill be at from 10 to 80 percent of the vessel volume, preferably from50 to percent thereof. Chlorine, fed from a tank of compressed gas 41 ofa source of chlorine such as chlorine hydrate, and mixed recirculatingchlorine, which may come from several batteries, will be added to thevessel at the bottom thereof through spargers 55, which will be made ofporcelain, glass frit, synthetic organic polymeric plastic, or otherchlorine gas and zinc chloride solution resistant materials. At leastone of such spargers will be located near the outlet of the saturatedelectrolyte being returned to the secondary battery or batteries. Sizesof the bubbles from the sparger will prefer ably be about 50 to 1,000microns, e.g., 500 microns average, for best solubility in theelectrolyte. In some cases, rather than have the recirculated chlorinemixed with the chlorine charged, it may be cycled back throughparticular sparger(s) and the rest of the chlorine to be added to thesystem may be directd through other sparger(s). However, generally it ispreferred to blend the two sources of chlorine together before bubblinginto the electrolyte. The mixing effect in the saturating vessel may beincreased by having additional stirrers or circulators located thereinbut this is not usually necessary. The velocity of the gas being passedthrough the spargers will be sufficient to produce a mixing effect inthe vessel to keep all of the electrolyte solution in motion, avoidingany dead spots in the vessel and thereby promoting solubility of thechlorine gas. Normally too, the electrolyte removed from the vessel willbe completely saturated with chlorine. The velocity of the gas from thespargers will generally be from 1 to 1,000 centimeters per second,preferably from to 500 cm. per second. Also, best operating conditionsfor solubility are in the temperature range of 20 to 70C. and in thepressure range of 0.5 to 10 atmospheres, greater pressures not beingdesirable for operation of the battery. Preferred temperatures are inthe range of 1 0 to 20C. and such pressures are at about l atmosphereilO percent. In most operations it is found that the portion of chlorinerecycled will usually be in the range of 5 to 25 percent of that spargedinto the elec: trolyte and preferably, this will be from 5 to percent.It is still desirable, however, to maintain the preferred volume ofelectrolyte in the vessel at 50 to 75 percent to provide sufficientvolume for absorption and to prevent a carryover of the liquid into thechlorine recycle duct.

In addition to efficiently resaturating the zinc chloride solution withchlorine, which solution is normally at a zinc chloride concentration of10 to 35 percent depending on the temperature thereof, the presentapparatus and method allow for efficient operation in conjunction withhigh energy density batteries of the zinczinc chloride-chlorine type,whether joined together and all operating, or whether only a fewbatteries or a single battery is returning depleted electrolyte to thevessel or being resaturated in a duct or passageway. Automatic controlsmay be employed to maintain the proportion of chlorine flow compared toelectrolyte flow or such controls may be manual, as by adjustment ofvalve 43. The apparatus and method are not sensitive to changes indemands and can withstand prolonged down periods without harm to theunit or parts of it. In fact, upon startup, due to the location of thechlorine spargers near the exit point for resaturated electrolyte to bereturned to the batteries, the initial electrolyte sent through thebatteries will have dissolved chlorine in it and its concentration ofchlorine will quickly increase.

The following examples illustrate but do not limit the EXAMPLE 1 Usingan apparatus of the type illustrated in the draw ing, with 16 spargersevenly distributed over a bottom of the cylindrical vessel and forcingchlorine gas at a pressure of 20 millimeters of mercury through thespargers into the vessel filled with depleted electrolyte circulatingfrom a discharging high energy density battery of bank of suchbatteries, containing about percent of zinc chloride in water, thesolution is raised to its operating concentration of dissolved chlorine.Thus, whereas the material charged to the saturating vessel contains aslittle as volume percent of chlorine, that discharged contains about 70volume percent or more of chlorine, on the basis of the volume of theelectrolyte. At higher absolute pressures in the vessel, the head ordriving force on the chlorine will be about the same but the content ofdissolved chlorine in the product will be greater. In addition to thedepleted electrolyte, which may have some chlorine gas with it, and inaddition to the chlorine directly charged to the spargers from a sourceof pressurized chlorine or chlorine hydrate, some of the chlorineutilized is recycled from the volume on top of the electrolyte solution.In the present example, the proportion of recycle is about 10 percent.

The flow of electrolyte into the saturating vessel is at the rate ofabout 15 liters per minute per 50 volt battery and when the resaturatoris employed for six batteries, the flow will be liters per minute. Thevolume of liquid in the vessel is such that there is a complete changewithin 10 seconds to a minute and, in the present example, in about 30seconds or less. Due to the finely divided bubbles of chlorine and thegood contact with the electrolyte obtained, the electrolyte produced issufficiently saturated with chlorine. For best operation, the velocityof the gas through the spargers is about 200 centimeters per second andthe temperature of the gas and electrolyte is about 0C. Because theoperating temperature of the batteries is usually about 30C. (althoughthis may be varied somewhat), it is generally desirable to utilizerefrigerating means, not shown in the drawing, to lower the electrolyteand gas temperatures for speediest saturation. However, satisfactoryresaturation is obtainable at 30C. with the present apparatus, followingthe methods described herein.

The spargers employed are constructed of glass frit, fused together andprotected by a perforated cover plate. At least one sparger is locatednear the outlet from the saturator, to improve countercurrent contactwith the electrolyte and better dissolve the chlorine. The sparger fritis of such size that at least one dimension thereof is in the l tomicrons range and the passages made between the pieces of fused glassaverage about lO to 20 microns. The spargers employed are 5 cm. indiameter and each carries a proportionate volume of the chlorine beingadded to the electrolyte. The bubbles produced by directing the thinstreams of chlorine gas through the sparger openings grow to diametersranging from 50 to 1,000 microns according to the combined effects ofcontact angle, gas velocity and severity of agitation of theelectrolyte.

After having been resaturated with chlorine, the electrolyte is returnedto the high energy density battery, in this case a zinc-zincchloride-chlorine battery, wherein it causes the generation of an opencell voltage of 2.1 volts per cell, or 1.7 volts at 8 amperes per cell.The cells are each of about 250 square centimeters effective workingarea on each of the active bipolar electrodes.

In modifications of this method, instead of aqueous zinc chlorideelectrolyte, other metal chloride electrolytes are employed, e.g.,chlorides of iron and nickel, and mixed metal halide solutions, and thesame desirable results are obtained. When the pressure of the system israised to five atmospheres, a more highly saturated solution is producedand saturation is achieved in less than half the time otherwise taken.

EXAMPLE 2 The method of Example l is followed with the exceptions that:the protective cover plate on the sparger surface is removed;essentially no chlorine gas is charged to the saturating vessel with theelectrolyte; the makeup chlorine is derived entirely from chlorinehydrate; the proportion of recycled chlorine in the vessel is 20percent; and additional mechanical agitation means (not shown in thedrawing) are employed to promote intimate contact of the chlorinebubbles with the electrolyte. Also, the spargers are tilted so as tohave half of them directed against the walls of the vessel to avoid anydead spots therein and the driving force for the gas is increased sothat the linear velocity thereof exiting from the spargers is about 500cm./second. Under such conditions, utilizing the same vessel and thesame spargers previously employed, saturation is reached in about halfthe time.

In preferred embodiments of the invention, such as those illustrated inthis example, automatic controls will be utilized to regulate thetemperatures, pressures, feed rates and agitation, so that theresaturated electrolyte or other aqueous solution will have the correctamount of chlorine dissolved therein and will enter the high energydensity battery at the desired temperature.

The invention has been described with respect to illustrations andexamples thereof but it is clear that it is not to be limited to thesebecause equivalents may be substituted for elements or steps in theinvention without departing from the spirit of the invention or goingbeyond its scope.

What is claimed is:

l. A method for preparing a chlorine saturated aque- I ous zinc chlorideelectrolyte for use as an electrolyte in an electrical energy storagedevice having a source of chlorine and an electrode compartment with atleast one positive and one negative electrode therein, said electrodecompartment having an inlet means and an outlet means, comprising thesteps:

1. passing chlorine from the source of chlorine to an inlet duct locatedin the bottom of a vessel having an aqueous zinc chloride solutiontherein; said vessel having an outlet means located near the bottom ofthe vessel, which outlet means is in communication with the electrodecompartment inlet means,

and an electrolyte solution inlet means in communication with theelectrode compartment outlet means, said solution inlet means beingspaced apart from the outlet means;

2. bubbling the chlorine through the inlet duct to the zinc chloridesolution at a velocity of from 1 to l,000 centimeters per second, sothat bubbles will form, ranging in size from about 50 to 1,000 microns,resulting in a zinc chloride solution saturated with chlorine;

3. removing undissolved chlorine from the vessel; 7

4. mixing the chlorine from Step No. 3 with additional chlorine from thechlorine source and passing it to the inlet duct;

5. passing the chlorine saturated zinc chloride solution from the outletmeans of the vessel to the electrode compartment inlet means;

6. passing the zinc chloride from the electrode compartment outlet meansto the solution inlet means in the vessel; and

7. maintaining the zinc chloride in the vessel at a level below that ofthe solution inlet means.

2. A method according to claim 1 wherein the temperature of the zincchloride solution charged to the volume of moving zinc chloride solutionis from 20 to C., the pressure is from 0.5 to 10 atmospheres, thesaturated zinc chloride solution is employed as a feed to a high energydensity secondary battery and the unsaturated zinc chloride to besaturated is from the battery.

3. The method according to claim 1 wherein the aqueous solution of zincchloride to be saturated with chlorine is from the effluent of a batteryor plurality of batteries, and the chlorine gas is from chlorine hydratewhich decomposes to chlorine and water.

2. bubbling the chlorine through the inlet duct to the zinc chloridesolution at a velocity of from 1 to 1,000 centimeters per second, sothat bubbles will form, ranging in size from about 50 to 1,000 microns,resulting in a zinc chloride solution saturated with chlorine;
 2. Amethod according to claim 1 wherein the temperature of the zinc chloridesolution charged to the volume of moving zinc chloride solution is from-20* to 70*C., the pressure is from 0.5 to 10 atmospheres, the saturatedzinc chloride solution is employed as a feed to a high energy densitysecondary battery and the unsaturated zinc chloride to be saturated isfrom the battery.
 3. The method according to claim 1 wherein the aqueoussolution of zinc chloride to be saturated with chlorine is from theeffluent of a battery or plurality of batteries, and the chlorine gas isfrom chlorine hydrate which decomposes to chlorine and water. 3.removing undissolved chlorine from the vessel;
 4. mixing the chlorinefrom Step No. 3 with additional chlorine from the chlorine source andpassing it to the inlet duct;
 5. passing the chlorine saturated zincchloride solution from the outlet means of the vessel to the electrodecompartment inlet means;
 6. passing the zinc chloride from the electrodecompartment outlet means to the solution inlet means in the vessel; and7. maintaining the zinc chloride in the vessel at a level below that ofthe solution inlet means.